Can you add PSNU-RM remote meter to POWER DOCK after initial sale

No.  The PSNU-RM must be ordered with a PWD2C at the same time.  The critical components for the PSNU-RM to function are only installed inside the PWD2C if a remote is ordered with it. 

The PWD2C cannot be retrofit to install in the field to accommodate the PSNU-RM after shipment.